I bought a new motor for my 1959 Land Rover about fifteen years ago because the one on there (possibly original) was sounding a little graunchy and sticking sometimes - not sure what make the new one was, got it from a Land Rover specialist. I never fitted it and it went with the vehicle unused when I sold it. It was a daily driver and on the odd occasion (once every couple of months) when it stuck, knocking it from underneath with the trolley jack handle always unstuck it.
I've only ever had inertia starters on Minis and they've never let me down. All my other cars apart from the Land Rover have had pre-engaged starters and they've never let me down either. Maybe that shows the quality of original equipment motors. I've no experience of non-genuine replacements apart from the Land Rover one I never fitted.
My Mini should have the Lucas M35J motor but it looks like an earlier motor and has a 10 tooth pinion instead of 9. Does anyone overhaul their motors anymore? I see brush kits on Ebay but are they any good?
One advantage of the pre-engaged motor is it overcomes the problem of the pinion being ejected during an isolated firing stroke - that can be annoying with an inertia motor.